Chirality-directed self-assembly of supramolecular propellers of dialkyltartrate trimers with hydronium, ammonium and primary aminium ions in CI mass spectra

The specific non-covalent gas phase association of chiral dialkyltartrates M with multiprotic onium ions A · H+, such as hydronium, ammonium and primary aminium ions, was investigated by chemical ionization (CI) mass spectrometry. The method applies enantiomer-labeled racemates of dimethyl-(S)- and [2H6]dimethyl-(R)-tartrate (s and r) and diisopropyl-(S)- and [2H14]diisopropylyl-(R)-tartrate (S and R). Unusually stable trimer adduct clusters M3 · AH+ were observed (5%–50% of the substrate ions). The abundances of the four diastereomers rrr · AH+, rrs · AH+, rss · AH+ and sss · AH+ did not show the statistics 1:3:3:1, but a pattern ranging from 1:1:1:1 to 5:3:3:5. Thus the homochiral clusters rrr · AH+ and sss · AH+ are three to five times more favored than the heterochiral ions. For the primary amines 1–10 as base A the chiral effect mainly depends on the CI conditions and hardly on the amine properties (proton affinity, steric crowding, chirality). For ammonia and water as base A the chiral effect was 2.1 and 2.7–3.6 respectively. The effect was not found in M3 · AH+ with diprotic onium ions from alcohols and secondary amines. It is assumed that the homochiral clusters M3 · AH+ have an especially stable supramolecular propeller structure as supported by PM3 calculations. The clusters allow CI experiments for molecular and chiral recognition.